While various techniques have been explored for detecting seeds falling in a tube, the most commonly used technique is that of a light source and photo sensor which detect the passage of seeds by counting pulses generated by the momentary reduction in light intensity from the steady state intensity caused by the passage of a seed between the light source and the sensor.
The vast majority of products of this type actually manufactured and sold in the marketplace have been manufactured by Dickey-John Corporation and they have obtained issue of a number of patents in this field. In particular the following patents of Dickey-John have some relevance in this field.
U.S. Pat. No. 3,723,989 (Fathauer) discloses an arrangement in which the intensity of the light source is varied to accommodate changes in reception by the sensor due to dirt or dust collecting between the light source and the sensor.
U.S. Pat. No. 3,974,377 (Steffen) discloses an arrangement in which the diode transmits light across the duct to two phototransistors which are connected in parallel to generate pulse signal information across a resistor where it is amplified and filtered.
U.S. Pat. No. 4,555,624 (Steffen) discloses a technique for analyzing the pulses generated by the momentary change in intensity value by following the changes in direction in the pulse and by generating individual positive and negative square-wave pulses in response to each change in direction.
U.S. Pat. No. 4,307,390 (Steffen) discloses an arrangement including a plurality of light sensitive sensors and an arrangement in which the number of counts or pulses recorded is increased relative to the number of input signals in dependence upon which one or ones of the plurality of light sensitive sensors are activated and in dependence upon the rate at which the signals are produced.
U.S. Pat. No. 4,166,948 (Steffen) discloses an arrangement in which the amplitude of the pulses is maintained above a predetermined minimum value by the sensor circuit, despite reductions in light intensity due to the accumulation of dirt and dust.
U.S. Pat. No. 5,635,911 (Landers) discloses an algorithm for calculating a seed count from pulses generated by the photo-transistor.
U.S. Pat. No. 4,491,241 (Knepler) a plurality of sensor circuits coupled in series circuit, one of the sensor circuits being coupled intermediate each of the sensors and a common signal line.
As set out in the above patents, one problem which arises in relation to seed counters of this type relates to the accumulation of dust and dirt which can vary the intensity of light normally received by the sensor.
A yet further problem relates to the geometry of the light source and the sensor which can either leave areas of the duct which are not properly monitored or can generate spurious reflections thus distorting the pulses.
Another arrangement which has become widely used is that manufactured by the present Assignees which is shown and described in U.S. Pat. No. 5,883,383 (Dragne) issued Mar. 16, 1999, U.S. Pat. No. 5,969,340 (Dragne) issued Oct. 19, 1999. Another patent relevant to this field is U.S. Pat. No. 6,661,514 (Tevs) issued Dec. 9, 2003.
The sensor unit it typically mounted on a seeder in the seed delivery duct at a position thereon downstream of the metering system and spaced from the discharge end of the duct. One problem which has arisen with arrangements of this type has been that of dust arising from the discharge end of the duct. This problem is particularly exacerbated by the use of so-called vacuum metering systems of the type shown for example in U.S. Pat. No. 7,152,542 (Eben) issued Dec. 26, 2006 to Kinze. This discloses an air seed meter for an agricultural planter, which has a vacuum chamber defining central opening, where atmospheric pressure is applied to vacuum side of the seed cells, when a seed orifice passes beneath the terminating end wall of the vacuum chamber.
Dust is stirred up through the process of planting and this can generate a signal similar to a seed resulting in a false count. This is due to the fact that dust moving through the tube may block a similar percentage of the light transmitted from the emitter to the detector as would be blocked by a seed. With the current design shown in the above patents of the present Assignees, a seed passing through the beam might obstruct 5% of the light. This reduction in photocurrent is amplified and if of sufficient amplitude, will produce a seed pulse. If a puff of dusty air with 95% transmissivity moves through the tube with similar passage time to a seed, the result will be a signal which is virtually indistinguishable from that produced by a seed falling through.